Indole can be produced by bacteria as a degradation product of the amino acid tryptophan.

Indole is an aromatic heterocyclic organic compound. It has a bicyclic structure, consisting of a six-membered benzene ring fused to a five-membered nitrogen-containing pyrrole ring. The participation of the nitrogen lone electron pair in the aromatic ring means that indole is not a base, and it does not behave like a simple amine. Indole is a solid at room temperature. Indole can be produced by bacteria as a degradation product of the amino acid tryptophan. It occurs naturally in human feces and has an intense fecal odor. At very low concentrations, however, it has a flowery smell, and is a constituent of many flower scents (such as orange blossoms) and perfumes. It also occurs in coal tar. The indole structure can be found in many organic compounds like the amino acid tryptophan and in tryptophan-containing protein, in alkaloids, and in pigments.

Indole undergoes electrophilic substitution, mainly at position 3. Substituted indoles are structural elements of (and for some compounds the synthetic precursors for) the tryptophan-derived tryptamine alkaloids like the neurotransmitter serotonin, and melatonin. Other indolic compounds include the plant hormone Auxin (indolyl-3-acetic acid, IAA), the anti-inflammatory drug indomethacin, and the betablocker pindolol. The name indole is a portmanteau of the words indigo and oleum, since indole was first isolated by treatment of the indigo dye with oleum.

Indole-3-carbinol is produced by the breakdown of the glucosinolate glucobrassicin which can be found at relatively high levels in cruciferous vegetables. Indole-3-carbinol is the subject of on-going biomedical research into its possible anticarcinogenic, antioxidant, and anti-atherogenic effects. Research on indole-3-carbinol has been conducted primarily using laboratory animals and cultured cells. Limited and inconclusive human studies have been reported. A recent review of the biomedical research literature found that, "evidence of an inverse association between cruciferous vegetable intake and breast or prostate cancer in humans is limited and inconsistent" and "larger randomized controlled trials are needed" to determine if supplemental indole-3-carbinol has health benefits.

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